Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a fax machine, or a multifunctional peripheral thereof, and more particularly, to a light scanning apparatus to be used in the image forming apparatus.
Description of the Related Art
As a light scanning apparatus to be used in an electrophotographic image forming apparatus, a light scanning apparatus having the following configuration is well known. Specifically, there has been known a light scanning apparatus configured to form an electrostatic latent image on a photosensitive member by deflecting a light beam emitted from a light source with a rotary polygon mirror and guiding the deflected light beam onto a photosensitive surface of the photosensitive member with optical components such as a lens and a mirror. An imaging optical system includes at least one fθ lens. The fθ lens has a special lens effective surface typified by an aspheric surface for the purpose of enhancing scanning characteristics. Further, a housing (hereinafter referred to as “optical box”) for supporting and fixing members of an optical system is formed by resin molding due to the advantages such as securement of a degree of freedom of a shape, a reduction in weight, and a reduction in cost. In particular, use of an optical box made of a resin is greatly advantageous in a tandem type image forming apparatus, because the number of optical components to be used is large, and further mounting directions and mounting methods for supporting and fixing the optical components are not uniform. On the other hand, compared to an optical box made of a metal, the optical box made of a resin has a large expansion coefficient under the condition of an increased temperature. Further, compared to the optical box made of a metal, the optical box made of a resin has a low thermal conductivity. Therefore, when the optical box made of a resin is used, in the light scanning apparatus containing a heat source, a temperature distribution of the optical box itself becomes non-uniform, and a hot portion and a cold portion occur partially. As a result, warpage and local distortion in different directions occur in the optical box.
When the rotary polygon mirror is rotated, the light scanning apparatus is increased in temperature due to heat generated in a driving portion such as a motor. When the rotary polygon mirror is driven by the motor continuously for a long period of time, an IC chip and the like mounted in the light scanning apparatus to drive the motor and the rotary polygon mirror are increased in temperature. Moreover, even when the rotary polygon mirror is driven by the motor for a short period of time, temperatures of the motor and the IC chip fluctuate to increase logarithmically immediately after the start of the rotation of the motor. Therefore, the optical box is greatly distorted and deformed due to a biased increase in temperature in the light scanning apparatus. A lens, the rotary polygon mirror, a mirror, and the like forming the light scanning apparatus are contained in the optical box, and hence the attitudes of optical components such as the lens and the mirror change due to the deformation of the optical box, with the result that the route through which a light beam passes and a reflecting direction vary due to the deformation of the optical box passage of time.
As described above, due to the deformation of the optical box, light beams of stations of respective colors vary in different directions and in different amounts, which changes a light-condensing position on a surface to be scanned and thus varies a horizontal direction, a vertical direction, or a magnification of an image line, resulting in the degradation in image. In particular, in a tandem type image forming apparatus, a light beam position of each color varies, and hence color misregistration occurs when toner images of respective colors are superimposed. In order to solve the foregoing problem, there has been proposed a light scanning apparatus in which hot current of air generated from a rotary polygon mirror and the like is diffused along an inclined surface by forming a rib provided perpendicularly to the bottom portion of a housing main body in an inclined manner (Japanese Patent No. 4170736). There has also been proposed a method in which a temperature detecting unit is provided in the light scanning apparatus, a temperature is detected by the temperature detecting unit, a color misregistration amount is predicted based on the detected temperature, and color misregistration is corrected based on the predicted color misregistration amount.
However, in the method in which the temperature in the light scanning apparatus is detected by the temperature detecting unit and the color misregistration amount is predicted and corrected based on the detected temperature, predictive control of the color misregistration amount with an even higher accuracy is required along with an increase in image quality of the image forming apparatus.